The present invention relates to a protection circuit for a switched mode power supply, which comprises a switching transistor and a transformer with a primary winding and an auxiliary winding for providing a supply voltage for driving the switching transistor.
A switched mode power supply of this kind is known for example from EP-A-0 936 726, which discloses self oscillating switched mode power supplies operating in a flyback mode. Specifically, FIG. 2 of EP-A-0 936 726 shows a switched mode power supply with a MOSFET as a switching transistor and a gate driver circuit comprising a charge capacitor being coupled via a diode to an auxiliary winding for providing a supply voltage for the gate driver circuit. It comprises further a threshold circuit, which keeps the gate driver circuit turned off after the switching-on of the switched mode power supply, until the supply voltage of the driver circuit has exceeded a specific threshold value with respect to the auxiliary winding. It comprises also an opto-coupler for providing a regulating voltage from the secondary side of the power supply for the gate driver circuit for stabilizing output voltages.
A switched mode power supply as described before, which contains a protection circuit, is shown in FIG. 1. It comprises a switching transistor T20, which is connected with its current input to a primary winding W1 of a transformer, not shown, and with its current output via a resistor R20 to ground. The primary winding W1 is coupled in the usual manner to a rectifying element BR, which provides a rectified voltage from the mains voltage UM.
The transformer comprises further an auxiliary winding W2 for providing a supply voltage Vcc. The voltage Vcc is a rectified and smoothed voltage, rectified via diodes D25 and D26 which are connected to a terminal 3 of the auxiliary winding 2, and stabilized via capacitors C25 and C24 and a Zener diode D27. The power supply comprises further a start-up circuit with resistors R5, R6, R7 being connected to the capacitors C25 and C24 and to the rectifying element BR, and a gate driver circuit GD.
The gate driver circuit GD controls a transistor T25 which is coupled to the supply voltage Vcc for providing a switching-on voltage to the control electrode of the switching transistor T20 via resistors R21, R27 for switching through this transistor. Via the gate driver circuit GD the output voltages of the power supply are stabilized, a control circuit of this kind is explained for example in EP-A-0 936 726.
The base of transistor T25, together with the gate driver circuit GD, is connected to a threshold circuit with a Zener diode D23 and resistors R24, R26 for keeping the transistor T25 off after switching-on of the power supply, until the supply voltage Vcc has reached a sufficiently high voltage value for the start-up of the power supply. The power supply comprises further small capacitors C23 and C26 to suppress voltage spikes of the corresponding diodes.
The Zener diode D27 of FIG. 1 provides also an over voltage protection for the power supply. In a failure mode, when the output voltage at the auxiliary winding W2 shoots up, the voltage Vcc at capacitors C24, C25 will also rise accordingly, but is clamped by the Zener diode D27, in this embodiment to 20 Volts. Therefore, the current flowing through Zener diode D27 will increase which will finally kill this diode when reaching a certain current value. Once the Zener diode D27 is destroyed, it will become short circuited, which will shut off the switched mode power supply immediately because the supply voltage Vcc drops.
The switched mode power supply of FIG. 1 has also an underdrive protection, which ensures that there is always a sufficient gate drive for switching through the switching transistor T20 even during a failure condition. This requirement is necessary due to safety reasons. This problem is solved in that two diodes D25, D26 and two capacitors C24, C25 are arranged each in parallel, to provide always a supply voltage Vcc, even when one of the diodes or one of the capacitors is open circuited.
The over voltage protection as described before is only working in switched mode power supplies with higher output power, for example 70 Watts, but not with switched mode power supplies limited to about 30 Watts, because then the current flowing through the Zener diode D27 is too low to destroy the Zener diode in case of an over voltage failure mode. In this case the output voltages of the power supply will continue to rise and this will damage the respective components of the device, for example capacitors and integrated circuits.
The object of the present invention is therefore, to provide a protection circuit, which works also reliably within a low power switched mode power supply, and to provide a respective switched mode power supply.
The protection circuit of the present invention comprises a clamping circuit with a switching element for reducing said supply voltage in case of a failure, especially in case of an overvoltage condition, and a holding circuit for providing a holding current for said clamping circuit. With the holding current the switch-off time in case of a failure is extended which avoids therefore an immediate start-up of the power supply when the supply voltage is reduced, and which would lead to a hiccup stage.
The clamping circuit comprises advantageously a threshold circuit, which provides a switching voltage for the switching element when the supply voltage reaches an upper voltage limit. The switching element is connected to a charge capacitor which provides the supply voltage for the switching transistor, and by reducing this supply voltage the switching of the switching transistor is disabled.
The holding circuit comprises in particular a capacitor which is coupled via a resistor to the clamping circuit, especially to the threshold circuit, for providing an additional current also in a failure mode. In a special embodiment the capacitor of the holding circuit is connected via a rectifying element to an auxiliary winding of the transformer, but can be coupled also to another input voltage of the power supply.
The protection circuit may be used especially for a switched mode power supply comprising a transformer with a primary winding and an auxiliary winding, and a switching transistor which is coupled to said primary winding and which is operating in a flyback mode. It works not only in power supplies with low power applications but also in power supplies providing high output power. For smaller switched mode power supplies, it is especially advantageous to use a MOSFET as a switching transistor and the clamping circuit therefore disables directly the gate drive of the MOSFET in case of an over voltage condition. Low power supplies for example with an output power of 30 Watts can be used for example as a second power supply within appliances of consumer electronics products.